This invention relates to improved noise suppression mixer nozzles for high bypass ratio jet propulsion engines and, more particularly, to a novel means for increasing the thrust efficiency of such engines without diminishing the noise reduction effectiveness of mixer nozzles used therein.
Noise suppression mixer nozzles generally of the configuration to which the present invention applies are well known in the art. In such nozzles the primary flow consisting of heated combustion product gases and excess air passing through the combustion chamber are confined by an inner duct that terminates in a multilobed discharge nozzle. The secondary flow consisting of ram air and fan driven air surrounding the inner duct is confined by a surrouding outer duct. As the outer duct converges towards the discharge end of the engine and the inner duct undergoes its transition into the multilobed discharge nozzle, the annular secondary flow column is divided by the nozzle lobes into fan-shaped segments as the primary flow spreads outwardly in the nozzle lobes in the spaces between such segments. As the interdigitated or interspersed primary and secondary flow patterns leave the exit plane of the inner duct nozzle, accelerated mixing occurs, producing lower noise within the converging after portion of the outer duct before discharging as a resultant thrust jet from the latter.
Such forced mixing low noise turbofan engine nozzles have received considerable attention in recent years as a means to augment thrust and thereby conserve fuel in high bypass engines. As it happened, however, performance levels were well below theoretical hopes and expectations. Whereas noise reductions were achieved by such nozzles beyond those attained with most conventional nozzles, it was found that lengthening of the nozzle sufficiently to accommodate what was regarded as a sufficiently slow rate of flow expansion in the diverging inner duct nozzle lobes intended to achieve efficient attached flow patterns imposed such high weight and drag penalties as to virtually eliminate most of the gains theoretically expected from the lengthened nozzles.
These and other parameter juggling attempts to overcome such limitations and difficulties proved to be relatively ineffective. In each instance, the completeness of mixing, hence the degree of noise reduction achieved, remained below anticipated levels, and energy losses remained much higher than expected based on theoretical predictions. Attempts to compromise in favor of one objective left shortfalls in attaining the other.
The present invention provides a means to utilize such mixing nozzles to maximum advantage (i.e., maximum noise suppression, maximum fuel efficiency); and indeed to achieve actually better than predicted performance levels based on calculated expectations applied to a given basic set of nozzle design parameters (i.e., lobed mixer length, diametral span, number and configuration of lobes, outer duct relative size and configuration, etc.). It also incidentally provides a means to appreciably increase nozzle life and makes possible achieving these and related objectives using established nozzle construction methods and materials without adding anything to weight (indeed permits reducing the weight), bulk, complexity or cost. It is particularly important in its usefullness with high bypass ratio engines and with nozzles of high diameter-to-length ratios, although application of the principles involved is not limited to extreme designs.